The present invention relates to an improved fusion bonding method and apparatus employing shear stress movement of the heat softened interface material at the fusion bonding interface of plastic elements.
The development of various plastic materials has resulted in a development of various methods of joining plastic elements to each other. In addition to the use of adhesives, fusion sealing and joining of plastic surfaces is well known wherein the surfaces are melted and joined under pressure to form a weld or fusion bond. The present inventor and assignee of this invention have been involved in the development of an unique fusion bonding of plastics and have developed various techniques for improving of a fusion bond involving plastic materials. For example, a particularly satisfactory method and system for internal heating of plastics for bonding is shown in U.S. Pat. No. 3,574,312 to Heller et al wherein discrete susceptor particles are introduced into the fusion interface material and subjected to a high frequency magnetic induction field. The particles react to the field to generate heat within the plastic material which is rapidly heated to a molten or softened and flowable state to permit a fusion bonding of the surfaces. As disclosed in the above and other patents, many factors and mechanisms must be considered in fusion bonding of plastic surfaces, including the particular plastic materials involved. For example, fusion bonding may be generally improved as disclosed in U.S. Pat. No. 3,941,641 to William C. Heller, Jr. et al by agitation or vibration of the molten material. A vibratory force is also advantageously employed to solve certain specific problems such as where air spaces and the like might be introduced into the area which would interfere with an adequate bond. Such a system as applied to a book binding application is disclosed, for example, in U.S. Pat. No. 3,925,126. The above as well as various other prior art discloses the many facets and difficulties associated with fusion bonding of plastics. Further, the development of particular plastic materials has created various particular problems. For example, the applicant's co-pending application entitled METHOD OF FUSION BONDING NON-ELASTOMERIC THERMOPLASTIC ELEMENTS WITH A BLOCK STRUCTURE ELASTOMERIC BONDING ELEMENT INTERPOSED AT THE BONDING INTERFACE with Ser. No. 832,970 which was filed on Sept. 13, 1977 discloses a particularly advantageous concept of employing a particular thermoplastic elastomer, generally having a dual olefin center block with styrene end blocks molecular structure, as a bonding agent for bonding of other dissimilar and nonelastomeric plastic elements. A particularly satisfactory material is a block polymer sold by Shell Chemical Company under the trademark Kraton G. Such elastomeric material also has properties which adapts it for production of various molded articles and as a conventional adhesive or hot melt sealant wherein it forms a physical attachment to other dissimilar surfaces to form a separate distinct interconnecting layer. Where such elastomeric material is used for the several parts of elements of a unit, difficulty has been found in providing a satisfactory fusion bonding of the like material together. The difficulty appears to be related to the molecular characteristic of the material. In the molding of such material, it is known that high shear rate molding methods should be employed. Thus, the molecular structure of the material is such that even though it is raised to a melting temperature, the viscosity of the melt is such that the material remains effectively as a solid, and does not gravity flow. However, by employing forced-flow methods, shear stress movement is created within the material which rapidly changes to a flowing state suitable for injection molding as well as transfer and extrusion molding processes. Shell Chemical Company has published literature indicating that the material Kraton G has a viscosity in the order of 100,000 to 500,000 poises, even with the material at or above its normal melting temperature. Typically for injection molding, a reciprocating screw machine is employed to move the material with a shear of the material which converts the material into a suitable flowing state for molding.
Fusion bonding of such material by heating to the melting temperature and applying pressure across the interface with a resulting flow of the material however does not result in a satisfactory fusion bond. A satisfactory or complete fusion bond is defined as one in which the material or elements do not separate by failure on the original plane, but rather by tearing along an unpredictable surface, as more fully discussed hereinafter. In fact, heating the material to a sufficient temperature level and time to otherwise effect any fusion bonding may result in charring the material. The failure to obtain a fusion bond between the two like high melt viscosity elastomeric materials is apparently because the molten or softened material, which does not flow laterally, does not, however, flow with the necessary shear characteristic even though under a clamping pressure and softened to a sufficient degree to otherwise form a true fusion bond and can be readily separated by hand printing at the interface.